A GPS (Global Positioning System) is a receiver that uses radio signals from special satellites to determine your location. They will work pretty much anywhere on the planet. But a GPS unit can do much more than tell you where you are it can lead you back to a location you’ve had the unit remember or lead you forward to waypoints you’ve determined from paper or electronic maps.

GPS units aid skiers and mountaineers in whiteouts, paddlers in fog, Search and Rescue personnel in relaying precise co-ordinates to aircraft, or folks who are just plain lost. However, since a GPS receiver may lose track of satellites, run out of power, or break, you should not rely solely on it for navigation. You should consider your GPS as your third tool the first two being a compass and a map.

GPS Features
There is a huge selection of handheld GPSs on the market. These are some of the features available:

WAAS Capability
WAAS stands for Wide Area Augmentation System, a set of satellites that broadcast additional signals to supplement those from the basic GPS satellites. A GPS that can receive WAAS signals is accurate to better than 3 meters, 95% of the time, compared to an accuracy of 15 meters without it. WAAS signals are currently available only in North America, but will soon be available in most areas of the world.

Weights and Screen Sizes
The larger the display screen, the bigger and heavier the unit is going to be. Hikers, who are usually holding the GPS in their hand (and carrying its full weight), will probably opt for the smallest, lightest unit possible. But paddlers or cyclists, who generally have their hands full and are reading the screen from a few feet away, may prefer a larger GPS that is legible from further away.

A larger screen only means a clearer image if the type size and icons used are larger. If possible, hold the GPS at the distance you’ll use and see if you can read it without squinting.

On Screen Displays
A typical GPS offers a number of “pages” to display different types of information. They include may include:

Navigation: These may take the form of a compass or arrow that points toward your destination waypoint as you move and / or a rolling highway that directs upcoming turns.

Maps: Almost all GPSs can display the relative positions of waypoints as a scattering of dots on a blank page (and your travel as a series of lines that connect those dots). Without displaying geographic features, this is not really a map. There are many GPSs that do offer true map pages, with contour lines, peaks, valleys, lakes, etc. These pages can show your position relative to such features in real time, without having to revert back to a paper map. To be truly useful, these maps must have a sufficient level of detail for your area of travel.

Satellite Position: A useful feature that displays all the satellites the GPS is currently receivingand the relative positions of all satellites that should be in view but are not currently being received. This can help you move around to unmask other satellites to get a position fix or improved accuracy.

Customizable Displays: Most GPSs can continuously calculate dozens of variables, including ETA (Estimated Time of Arrival), Average Speed, VMG (Velocity Made Good the speed that you are actually moving towards your destination). In practice, most people make use of only a few of these figures on a regular basis. A good GPS will have one or more customizable windows on the navigation pages that let you display the data you find most helpful.